1. Field of the Invention
The present invention relates to a photoelectric conversion device, image capturing system, and method of manufacturing a photoelectric conversion device.
2. Description of the Related Art
An active-pixel type photoelectric conversion device represented by a CMOS image sensor includes a pixel array in which a plurality of pixels are arranged in a matrix pattern, and an output circuit which processes a signal read out from the pixel array onto a signal line and outputs the processed signal. Some photoelectric conversion devices of this type have a global electronic shutter function in which all pixels in the pixel array start and end charge accumulation operations at the same timings.
Note that the CMOS image sensor cannot read out signals of pixels of all rows in the pixel array onto signal lines at the same time in terms of its structure. In a photoelectric conversion device having the global electronic shutter function, each pixel immediately transfers a charge accumulated by a photodiode (PD) to a carrier pocket (CP) in place of a floating diffusion (FD) at the end timing of the charge accumulation operation. A charge which is transferred to and held by the CP in each of pixels of each row is sequentially transferred from the CP to the FD for each row, and a signal according to the charge transferred to the FD is read out onto a signal line by an amplification transistor of each pixel.
Note that the CP has a role of holding a charge after completion of accumulation of a certain pixel until the charge is read out. Therefore, when light becomes incident on the CP and a charge is generated by photoelectric conversion at a PN junction which contacts the CP, leak noise of light is mixed in a signal held in the CP. As a result, the image quality of an obtained image deteriorates.
In order to solve the above problem, in Japanese Patent Laid-Open No. 2007-115803, the CP is covered by a first transfer gate electrode used to transfer a charge from the PD to the CP, and the first transfer gate electrode is covered by a second transfer gate electrode used to transfer a charge from the CP to the FD. Thus, according to Japanese Patent Laid-Open No. 2007-115803, intrusion of light into the CP can be suppressed.
With the technique described in Japanese Patent Laid-Open No. 2007-115803, a pattern of the first transfer gate electrode is formed above a semiconductor substrate, an insulation film is formed to cover the first transfer gate electrode, and a pattern of the second transfer gate electrode is then formed on that insulation film. With this manufacturing method, processes required to form the first and second transfer gate electrodes are complicated.